A typical method is known of causing flow of a sweep gas into the permeate-side of a gas separation membrane in a configuration in which a feed gas that contains two or more gases is supplied to a feed-side of a gas separation membrane and a predetermined gas contained in the feed gas permeates the permeate-side of the gas separation membrane (for example reference is made to Cited References 1 and 2). This method enhances the permeation efficiency by reducing the partial pressure of the permeate gas on the permeating side.